@Samantha,
I am guessing this is your first waiver?
On the waiver itself it does state "PERIOD OF TEMPORARY STAY: To be determined by the admitting officer."
In other words, the waiver comes from DHS, so unless you try to lie your way in, the officer in question can revoke the waiver by any reason he seems valid on his own suspicions, if flying by air just have all your destinations in order, hotel, transfers, friends or family numbers, etc etc... and you should be fine without secondary checks.
Same applies for driving through land.
I go thru every weekend, I have my stories straight and never had any major problems entering the states unless the CPB wants to just waste time, but make sure you and friends/family your travelling with or seeing in US have the same stories and after a brief time in the office they will let you go to your destination or send you back to Canada, its as simple as that.